1. Field of the Invention
The present invention relates generally to linear motor device and, more particularly, relates to an improved linear motor device having a vibration reduction unit. The present invention has particular applicability to copying machines.
2. Description of the Background Art
Linear motors have been widely used for various kinds of industrial equipment including electronic equipment requiring linear movement. While a linear motor can move a movable member to be carried in a quick and precise manner, vibration is caused by the movement. If the vibration is transmitted to another components within the equipment employing the linear motor, in some cases, a malfunction or a problem is caused to the components. Particularly, as electronic equipment such as a copying machine and optical or magnetic disc apparatus employing a linear motor encounters a serious problem caused by the vibration, various measures are taken in order to prevent or reduce the vibration.
For example, in optical or magnetic disc apparatus, a movable unit including a voice coil is mounted on a rectilinearly movable member and a magnetic field forming member including permanent magnet is fixed on a base frame. When the movable member is moved, the voice coil is driven and a thrust is caused at the movable unit. The magnetic field forming member receives reaction force corresponding to the thrust and the reaction force is transmitted to the base frame. As a result, vibration is caused in the base frame or, in some cases, the shape of the base frame is changed, which often causes malfunction in data access, i.e., data reading/writing in the optical or magnetic disc apparatus.
While the present invention is applicable to various kinds of equipment including a copying machine and optical or magnetic disc apparatus employing a linear motor, a description will be made of one example where the present invention is applied to a copying machine in the following.
FIG. 1 is a block diagram of a conventional copying machine. Referring to FIG. 1, the copying machine includes two movable members 10a and 10b which move linearly along an original document surface 64. There are provided on movable member 10a a halogen lamp 102 for directing light for reading out an image onto the original document and a reflecting mirror 101 for reflecting the reflected light from the original document. Two reflecting mirrors 103 and 104 are provided on movable member 10b, for reflecting light directed from reflecting mirror 101. The light reflected by reflecting mirror 104 is given to a reflecting mirror 66 through a zoom lens 65. The light reflected by reflecting mirror 66 is irradiated on a photoreceptor drum 67. It is noted that reflecting mirrors 101, 103, 104 and 66 are provided for changing the light path of the light supplied from the original document.
A developing unit 69 supplies toner to photoreceptor drum 67 according to the light directed on photoreceptor drum 67. Accordingly, a toner image 70 is formed on photoreceptor drum 67 and the toner image is transferred onto a recording paper 72, which is prepared in advance, by a transfer unit 71. The transferred image is fixed on recording paper 72 by a fixing unit 73. A discharging unit 74 eliminates electric charge left on photoreceptor drum 67. A cleaner 75 is provided for removing the toner left on photoreceptor drum 67. The surface of photoreceptor drum 67 is restored to its initial state by a charging unit 68.
FIG. 3 is a plan view of a copying machine representing the background of the present invention. The copying machine shown in FIG. 3 has been already proposed in the earlier application by the applicant of the present invention. Referring to FIG. 3, the copying machine includes a base frame 1, a magnetic field forming device 36 movable in the lateral direction, that is, the directions A and B, and movable members 10a and 10b each capable of moving independently from each other in the lateral direction. Movable members 10a and 10b correspond to movable members 10a and 10b shown in FIG. 1, respectively. Accordingly, movable member 10a shown in FIG. 3 includes a halogen lamp 102 and a reflecting mirror 101 (not shown). Movable member 10b also includes reflecting mirrors 103 and 104 (not shown). It is noted that these movable members 10a and 10b are moved with the velocity ratio of 2:1 in order to keep constant the light path length of the light reflected from the original document. Movable members 10a and 10b move in the direction B in the return operation after moving in the direction A in the scanning operation.
Two holders 2 are attached on base frame 1 and a rail 3 is held by holders 2. Movable member 10a moves along rail 3 through a bearing 11a. Movable member 10b also moves along rail 3 through a bearing 11b.
Movable units 20a are provided on both ends of movable member 10a. Similarly, movable units 20b are provided on both ends of movable member 10b. Magnetic field forming members 30a and 30b facing movable units 20a and 20b are provided within a magnetic field forming device 36. Linear encoders 13a and 13b are formed on movable members 10a and 10b, respectively. A stator scale 13c is attached on base frame 1 through two fixed members 5. Linear encoders 13a and 13b detect the present positions of movable members 10a and 10b on stator scale 13c.
Magnetic field forming member 30a includes a plate-like permanent magnet 32a and a fixed yoke 31a. Similarly, magnetic field forming member 30b includes a plate-like permanent magnet 32b and fixed yoke 31b. Magnetic field forming members 30a and 30b are coupled to each other by connecting plates 33 and 34. Accordingly magnetic field forming device 36 is formed of magnetic field forming members 30a, 30b and connecting plates 33, 34. Two bearings 35 for supporting guide rail 3 is provided inside magnetic field forming member 30a.
FIG. 2 is a structural cross-sectional view taken along the line V--V in FIG. 3. Referring to FIG. 2, a rail 4 is attached at the upper side of base frame 1 and a roller 12a runs on rail 4. Movable member 10a is connected to roller 12a by a connecting member 22a. A total of two rollers 6 are provided under magnetic field forming member 30b and on base frame 1, so that magnetic field forming device 36 can move in either direction A or B on base frame 1.
Movable unit 20a includes movable yokes 21a and 22a attached on both sides of movable member 10a and two three-phase coils 23a attached outside of movable yokes 21a and 22a. A three-phase brushless linear motor is made up of the two three-phase coils 23a and permanent magnets 32a and 32b facing three-phase coils 23a and formed inside fixed yokes 31a and 31b. Hall elements (not shown) are provided at predetermined positions on movable yokes 21a and 22a for detecting a timing for magnetization switching of three-phase coils 23a. The hall elements detect the change of magnetic fluxes of permanent magnets 32a, 32b. Similarly, another three-phase brushless linear motor is made up of two movable units 20b and magnetic field forming members 30a and 30b.
Referring back to FIG. 3, struts 41a and 41b are provided outside of fixed yoke 31a of magnetic field forming device 36. FIG. 4 is a structural cross-sectional view taken along the line W--W in FIG. 3. Struts 41a and 41b extend through two holes 42a and 42b formed in the side surface of the base frame. A pulley 45 on which a wire 43 having a predetermined tension is wound is provided between struts 41a and 41b and outside of base frame 1. Pulley 45, along with a timing pulley 46, is provided inside a gear box 47 attached outside of base frame 1. Pulley 45 and timing pulley 46 are connected to each other through a timing belt 48. A pulse motor 49 is provided outside of gear box 47 and a rotation shaft of pulse motor 49 is connected to timing pulley 46. As a result, magnetic field forming device 36 can be moved by driving pulse motor 49. In other words, wire 43, pulley 45, timing pulley 46, timing belt 48 and pulse motor 49 constitute a driving device 40 for driving magnetic field forming device 36.
Timing pulley 46 is rotated by driving pulse motor 49. As timing belt 48 transmits the rotation of timing pulley 46 to pulley 45, the rotating velocity of pulley 45 is changed with a predetermined reduction gear ratio or velocity reduction ratio. The rotation of pulley 45 transmits its power to magnetic field forming device 36 through wire 43 and struts 41a and 41b. Accordingly, magnetic field forming device 36 is moved in the direction A or B in base frame 1. The detail of driving device 40 in FIG. 3 is shown in FIG. 6.
FIG. 5 is a block diagram of a motor control circuit 50 and a motor driving circuit 60 for controlling and driving pulse motor 49 shown in FIG. 3. Referring to FIG. 5, there are provided in movable member 10a three-phase coil 23a as an armature winding, and linear encoder 13a for detecting the position of movable member 10a. Similarly, three-phase coil 23b and linear encoder 13b are provided in movable member 10b. Each positional detection signal supplied by linear encoders 13a and 13b includes two rectangular wave signals having a phase difference of 90.degree. each other and a home position detection signal. The positional detection signal is supplied to a microprocessor (MPU) 51 through waveform shapers 51a and 51b, and the present positions and moving velocities of movable members 10a and 10b are obtained in microprocessor 51.
Microprocessor 51 includes a memory (not shown) in which the movement positional data and velocity data of movable members 10a and 10b are stored in advance. Microprocessor 51 compares the stored positional data with the present positional data detected and supplies a three-phase driving signal for driving the linear motors to three-phase drivers 52a and 52b based on the result of the comparison. Three-phase drivers 52a and 52b supply three-phase driving signals for driving three-phase coils 23a and 23b in response to the driving signal supplied from microprocessor 51.
That is, motor control circuit 50 receives the positional detection signals supplied from linear encoders 13a and 13b as feedback signals and controls the positions and velocities of two linear motors independently from each other.
Motor driving circuit 60 includes an input selection switch 61, a logic circuit 62 and a four-phase current driver circuit 63. A count up pulse Up and a count down pulse Dn are supplied to one input S of switch 61 from motor control circuit 50. A count up pulse Upi and a count down pulse Dni in the initial state are provided to the other terminal I of switch 61.
When the power supply is turned on, switch 61 is connected to terminal I and magnetic field forming device 36 is moved to a position defined by the initializing signals Upi and Dni. As switch 61 is connected to terminal S after the initialization is completed, the count up pulse Up and the count down pulse Dn indicating the position of movable member 10a are supplied to logic circuit 62 through switch 61. A predetermined logic processing is carried out in logic circuit 62 and an output signal therefrom is supplied to four phase current driver circuit 63. Four phase current driver circuit 63 supplies driving currents of four phases to pulse motor 49 so that magnetic field forming device 36 is moved.
Movable members 10a and 10b are moved with the velocity ratio of 2:1 as stated above. The changes in the velocities Va and Vb of movable members 10a and 10b are shown in FIG. 7A. In FIG. 7A, the axis of abscissas represents the passage of time t and the axis of ordinates represents velocity V.
As movable members 10a and 10b are moved according to the velocity patterns shown in FIG. 7, the respective positions Xa and Xb of movable members 10a and 10b change as shown in FIG. 7B. In FIG. 7B, the axis of ordinates represents a distance X of movement with the direction A shown in FIG. 3 being positive.
As shown in FIGS. 7A and 7B, when the original document is scanned, the two movable members 10a and 10b are moved according to the velocity patterns and positional patterns shown in FIGS. 7A and 7B (the data is stored in the memory within microprocessor 51). The propulsion force for moving movable members 10a and 10b is obtained by the linear motors which have already been described. That is, as shown in FIG. 2, movable member 10a obtains the propulsion force from the two linear motors including magnetic field forming members 30a and 30b and three-phase coils 23a. Similarly, movable member 10b also receives another propulsion force. As movable members 10a and 10b receive the propulsion force from magnetic field forming members 30a and 30b through electromagnetic coupling, magnetic field forming members 30a and 30b also receive reaction forces from the two movable members 10a and 10b. Though the magnitudes of the reaction forces are the same as those of movable members 10a and 10b, they are applied in the opposite direction. Accordingly, magnetic field forming device 36 is moved by the reaction force.
FIG. 7C indicates a distance Y of movement of magnetic field forming device 36. The positive direction of movement of magnetic field forming device 36 in FIG. 7C is opposite to that of movable members 10a and 10b in FIG. 7B. As movable members 10a and 10b move in the scanning direction A in the scanning period Ts, magnetic field forming device 36 moves in the opposite direction B in this period. Conversely, as movable members 10a and 10b move in the direction B in the return period Tr, magnetic field forming device 36 is moved in the direction A by the reaction force. As a result, the reaction force Fr caused by the movement of movable members 10a and 10b is not transmitted to base frame 1, so that the vibration caused by the movement of movable members 10a and 10b is not transmitted to base frame 1.
A curved line Y1 shown in FIG. 7C is obtained assuming that there is no frictional force F.mu. in the movement of magnetic field forming device 36. Actually, as frictional force F.mu. (&gt;0) is caused between magnetic field forming device 36 and base frame 1 with the movement, magnetic field forming device 36 is moved according to the curved line Y2 in FIG. 7C. That is, as there is frictional force F.mu., which is not negligible, between magnetic field forming device 36 and base frame 1, magnetic field forming device 36 can not be returned to its original position only by the reaction force from movable members 10a and 10b.
Supposing that the propulsion forces of movable members 10a and 10b are Fa (t) and Fb (t), and frictional forces of movable members 10a and 10b and magnetic field forming device 36 are F.mu.a, F.mu.b and F.mu.m, respectively, forces F1 (t), F2 (t) and Fm (t) acting on movable members 10a and 10b and magnetic field forming device 36 are obtained in accordance with the following equations: EQU F1(t)=Fa(t)-F.mu.a (1) EQU F2(t)=Fb(t)-F.mu.b (2) EQU Fr(t)=Fa(t)+Fb(t) (3) ##EQU1##
In this case, if F.mu.a+F.mu.b-F.mu.m is defined as a friction load loss F.mu., the following relation holds true, so that the relation of F.mu.&gt;0 is obtained as movable members 10a and 10b are supported by plain bearings while magnetic field forming device 36 is supported by a ball-and-roller bearing: EQU F.mu.a+F.mu.b&gt;F.mu.m (6).
The velocity reduction ratio of the velocity reduction mechanism of pulse motor 49 shown in FIG. 6 is so adapted that the load of pulse motor 49 is 0 under the ideal condition (F.mu.=0). Supposing that x is the velocity reduction ratio of the velocity reduction mechanism and r is the radius of wire pulley 45, the torque T generated at the rotation shaft of pulse motor 49 is expressed by the following equation: EQU T=(1/x).multidot.r.multidot.F.mu. (7)
If F.mu.=0, the electrical angle of a field winding in pulse motor 49 always coincides with the rotor rotation angle of pulse motor 49 to which wire 43, pulley 45 and so on are coupled, so that no rotating force is caused in the rotor of pulse motor 49. That is, pulse motor 49 is operated under a no-load condition.
Conversely, if there is friction load loss F.mu., the electrical angle of the field winding of pulse motor 49 does not coincide with the rotor rotation angle of pulse motor 49, so that rotating force is generated in the rotor of pulse motor 49 in such a direction that the difference therebetween is reduced. The rotating force is transmitted to magnetic field forming device 36 through wire 43, pulley 45 and so on, so that magnetic field forming device 36 is controlled in the same way as in the case where F.mu.=0. Accordingly, also in the case where F.mu.&gt;0, magnetic field forming device 36 is returned to the exact, initial position after the scanning operation as indicated by the curved line Y3 shown in FIG. 8A.
In this way, magnetic field forming device 36 is always moved to a desired position by the action of pulse motor 49 in the copying machine shown in FIG. 3. The following problems, however, are pointed out. That is, as the friction load loss F.mu. becomes larger, a larger output torque is required for pulse motor 49. If the friction load loss F.mu. is excessively larger than the maximum output torque of pulse motor 49 (F.mu.&gt;&gt;0), a step-out is caused in pulse motor 49. As the driving frequency of pulse motor 49 changes as shown in FIG. 8B, the step-out is likely to be caused particularly in the latter half period Td in the return period. If the step-out is caused in pulse motor 49, magnetic field forming device 36 is not returned to the desired initial position as indicated by the broken line Y3' in FIG. 8A. As a result, in the next scanning operation, a problem is caused such as a collision of movable members 10a and 10b with bearings 35 or a collision of connecting plates 33 and 34 with holders 2.
In order to prevent the step-out of pulse motor 49, it is necessary to use, as motor 49, a pulse motor having a larger maximum output torque. A pulse motor having a larger output torque, however, is generally large, so that the size and cost of the copying machine is inevitably increased.